Nicene and Post-Nicene Fathers, Vol. V:A Work on the Proceedings of Pelagius.: Chapter 27

Pelagius was charged with having said: “That the Church here is without spot or wrinkle.” It was on this point that the Donatists also were constantly at conflict with us in our conference. We used, in their case, to lay especial stress on the mixture of bad men with good, like that of the chaff with the wheat; and we were led to this idea by the similitude of the threshing-floor. We might apply the same illustration in answer to our present opponents, unless indeed they would have the Church consist only of good men, whom they assert to be without any sin whatever, that so the Church might be without spot or wrinkle. If this be their meaning, then I repeat the same words as I quoted just now; for how can they be members of the Church, of whom the voice of a truthful humility declares, “If we say that we have no sin, we deceive ourselves, and the truth is not in us?” 1683
or how could the Church offer up that prayer which the Lord taught her to use, “Forgive us our debts,” 1684
if in this world the Church is without a spot or blemish? In short, they must themselves submit to be strictly catechised respecting themselves: do they really allow that they have any sins of their own? If their answer is in the negative, then they must be plainly told that they are deceiving themselves, and the truth is not in them. If, however, they shall acknowledge that they do commit sin, what is this but a confession of their own wrinkle and spot? They therefore are not members of the Church; because the Church is without spot and wrinkle, while they have both spot and wrinkle.